Not applicable.
Not applicable.
The present invention is directed to methods of producing materials from metallurgical powders and the materials provided by those methods. More particularly, the present invention is directed to methods of producing materials from metallurgical powders wherein the materials exhibit tensile strength greater than 100 ksi, yield strength greater than 80 ksi, Rockwell C (RC) hardness of at least 20, and elongation greater than 2%. Wrought materials commonly referred to as xe2x80x9cductile cast ironxe2x80x9d generally exhibit those properties and are commonly used in applications such as engine and transmission parts. Certain engine and transmission parts, such as ring gears and other transmission gears, require a combination of high tensile and yield strength to withstand the forces encountered in such applications, RC hardness of 20 or greater to provide suitable wear resistance with acceptable machinablity, and elongation greater than about 2% to ensure adequate resistance to impact loading. Powder metal materials may be prepared by methods within the present invention with ductile iron-like properties and may be used in certain applications as substitutes for conventional ductile cast iron parts. More particularly, materials may be prepared by methods within the present invention having tensile strength greater than 100 ksi, yield strength greater than 80 ksi, Rockwell C (RC) hardness of at least 20, and elongation greater than 2%
Ductile cast iron, also known as nodular iron or spheroidal-graphite cast iron, is cast iron that includes graphite in the form of tiny spheres or nodules. Because of additives introduced into the molten iron before casting, the graphite grows as spheres rather than as the flakes characteristic of gray iron. Ductile cast iron is much stronger and has higher elongation than gray iron, and it may be considered a natural composite in which the spheroidal graphite imparts unique properties to the material. Typical properties of ductile cast iron include tensile strength greater than 100 ksi, yield strength greater than 80 ksi, RC hardness of about 20, and elongation greater than 2%.
The relatively high strength and toughness of ductile cast iron provide advantages over gray in many structural applications. Such applications include, for example, automotive engine and transmission parts. Ductile cast iron can be produced to X-ray standards because porosity typically stays in the thermal center of the material.
Production of parts from ductile cast iron has many attendant difficulties. Greater metallurgical and process control is required in producing parts of ductile cast iron than in producing parts of other cast irons. Repeated chemical, mechanical, and metallurgical testing is needed to ensure that the required quality is maintained and that specifications are met. Producing ductile cast iron also requires carefull selection of charge materials, which must be free of undesirable residual elements. For example, carbon, manganese, silicon, phosphorous, and sulfur must be held at specified levels. Levels of magnesium, cerium, and certain other elements also must be controlled in order to attain the desired generally spherical graphite shape and to prevent the deleterious effects resulting from the presence of elements such as antimony, lead, titanium, tellurium, bismuth, and zirconium. The latter elements interfere with the graphite nodulizing process, and they must be either eliminated or restricted to very low concentrations.
An inherent limitation of ductile cast iron is that parts are produced of the material by casting. The casting of parts having intricate shapes is difficult, and significant machining or other finishing operations typically are required to prepare ductile cast iron parts in final form. Thus, although ductile cast iron provides significant advantages, primarily in terms of strength and elongation, it is difficult and time-consuming to prepare parts from the material on a commercial scale.
Accordingly, the need exists for methods of advantageously producing material and parts on a commercial scale having properties similar to ductile cast iron. A need also exists for materials having properties similar to ductile cast iron and which are produced by methods other than the casting of molten materials.
In order to address the above-described needs, the present invention provides a novel method for producing a material from metallurgical powder. The method may be adapted to provide materials having tensile strength greater than 100 ksi, yield strength greater than 80 ksi, Rockwell C (RC) hardness of at least 20, and elongation greater than 2%, properties typically exhibited by ductile cast iron. The method includes the step of providing a metallurgical powder that includes at least one low alloy steel powder and 0.3 up to 1.0, preferably 0.4 up to 0.65, weight percent carbon in elemental, alloyed, or another form. At least a portion of the metallurgical powder is molded under pressure to provide a compact, and the compact is sintered at a temperature in the range of 1800xc2x0 F. to 2400xc2x0 F. and below the melting temperature of the compact, preferably for 10-60 minutes, to bond together the powder particles and provide a sintered compact. The sintered compact is subsequently hot formed. The hot formed compact is heated to a temperature in the range of 1000xc2x0 F. to 2300xc2x0 F. and below the melting temperature of the compact to re-sinter the compact. The hot forming and the subsequent re-sintering steps may work in conjunction to provide the material with the ductile iron-like properties.
According to another aspect of the invention, the invention is directed to a method for providing a material from metallurgical powder wherein the method includes providing metallurgical powder that includes at least one low alloy steel powder and 0.4 up to 0.65 weight percent carbon in elemental, alloyed, or another form. A compact is formed by pressing at least a portion of the powder in a mold at 20 to 70 tsi. The compact is heated to a temperature greater than 1800xc2x0 F. and less than the melting temperature of the metallurgical powder to bond the metallurgical powder and form a sintered compact. The sintered compact is further compressed in a heated die at 20 to 80 tsi while the compact is at a temperature of 1400xc2x0 F. to 2000xc2x0 F., and the compact may then be cooled. The compact is subsequently heated to a temperature of 1000xc2x0 F. to 2300xc2x0 F. and below the melting temperature of the compact. The compact is then cooled to ambient temperature.
Additional aspects of the present invention are directed to materials produced by a method of the invention and articles of manufacture composed of or including such materials. Materials of the present invention find broad application and, as an example, may be used to form automotive engine and transmission parts such as, for example, ring gears and other gears, pinions, rollers, slides, valves, output shaft hubs, and reaction hubs.
As used herein, xe2x80x9cmetallurgical powderxe2x80x9d refers to a particulate material including one or a combination of several particulate metal-containing materials. Thus, a metallurgical powder as used herein may include constituents including, for example, pure iron powder, other pure metal powders, and/or iron alloy or other alloyed powders, and also may include additive powders such as graphite, lubricants, additives enhancing wear resistance, and other additives enhancing one or more properties of the final material. As used herein, xe2x80x9csinteringxe2x80x9d has the meaning generally ascribed to that term by those of ordinary skill in the art and generally refers to heating a material composed of powder particles at a temperature of at least 70% of and lower than the melting temperature of the material, typically in a protective atmosphere, for a time so as to bond the powder particles. In the case of iron-base powder metal parts, for example, sintering typically involves heating the parts at temperatures within the range of 1800-2400xc2x0 F. for 10-60 minutes. xe2x80x9cRe-sinteringxe2x80x9d conventionally refers to subjecting a powder metal part which has already been sintered to a second thermal cycle, typically, but not necessarily at for times and temperatures within the ranges of the earlier sinter. As used herein, a xe2x80x9clow alloy steel powderxe2x80x9d refers to particulate material composed predominantly of iron and including one or more alloying ingredient in individual amounts less than about 10% by weight of the particulate material
Material may be produced by a method of the present invention having properties similar to ductieast iron, but without a number of the attendant difficulties of producing the cast material. Powder metal material provided by the method of the present invention may have, for example, high tensile strength (greater than 100 ksi), yield strength in excess of 80 ksi, RC hardness of at least 20, and elongation greater than 2%. Thus, material produced by a method of the present invention may be used in many applications formerly filled by conventional ductile cast iron material or by other materials having the foregoing properties. Powder metallurgy production techniques utilized in the present method obviate many of the difficulties encountered with casting techniques. Also, because the properties of material produced by a method of the present invention do not depend on the presence of spherical graphite generated during freezing from the melt, the close control of material chemistry is not so critical.
The reader will appreciate the foregoing details and advantages of the present invention, as well as others, upon consideration of the following detailed description of embodiments of the invention. The reader also may comprehend additional advantages and details of the present invention upon carrying out or using the invention.